1. Field of the Invention
This invention relates to a method for using water soluble zirconium(IV) compounds to prevent pitch deposition in the pulping and papermaking processes. One aspect of this invention pertains to a method for preventing the deposition of pitch on machinery used in the pulping and papermaking processes. Another aspect of this invention pertains to a method of preventing the formation of visible pitch particles in the final paper of a papermaking process. Yet another aspect of this invention pertains to a method for removing pitch deposits from machinery used in the pulping and papermaking processes.
2. Description of the Related Art
The problems caused by the build up of pitch on pulp and papermaking machinery and in the final paper cost the pulp and paper industry more than 30 million dollars a year in lost production. Pitch is generally considered to be a resin-based deposit of widely varying composition originating in the extractive fraction of wood. The extractive fraction of wood is one of the four principal components of wood. The other three are cellulose, lignin, and hemicellulose. The extractive fraction is defined as a complex mixture of substances which are soluble in cold water, alcohol, benzene, ether, and acetone. The extractive fraction, which makes up from about 3% to 10% of the weight of wood, contains such components as low molecular weight carbohydrates, terpenes, aromatic and aliphatic acids, alcohols, tannins, color substances, proteins, phlobaphenes, lignins, alkaloids, and soluble lignins. Pitch is a major problem in pulp and papermaking because it (1) agglomerates and also occludes other matter to form visible "dirt" in the final paper, (2) plates out and collects on machinery used in the pulp and papermaking process such as screens, filters, refining equipment, pulp washers, and the paper machine, and (3) reduces pulp brightness and brightness stability. The composition and amount of pitch deposited on pulp and papermaking machinery and in the final paper varies with the time of the year the trees are harvested, the type of wood, and the type of the pulping process. For example, wood pulped from trees cut in the early spring and fall causes more pitch problems than wood from trees cut at other times during the year. Pitch deposited in softwood Kraft mills tends to have a relatively larger abietic acid to fatty acid-ester ratio than pitch found in hardwood Kraft mills. Pitch deposit problems are somewhat more severe in sulfite mills. The sulfite pulping process removes only about one half of the resins and fatty esters leaving a considerable portion of these materials encapsulated within the cellulose fibers. However, these encapsulated materials are released by the shearing forces of the refining process and thus pitch deposits are more prevalent in the stock preparation area and on the paper machine. Pitch problems can be quite bothersome in mechanical pulp mills, including groundwood, TMP, CTMP, and semi-chemical pulping processes, particularly those that utilize softwoods. This is because there is little chemical degradation of the fatty acid esters and resin esters. Therefore, those materials are not washed out and tend to remain dispersed in the aqueous system of the pulping process.
The presence of calcium carbonate in the pulping process exacerbates the problem of pitch deposition on pulp and papermaking machinery. Crystallized calcium carbonate can provide nucleation sites for precipitated metal soaps thereby producing hydrophobic particles which coalesce with other particles to form a pitch deposit.
There have been many attempts over the years to eliminate pitch problems by employing pitch control agents in the pulp and/or papermaking processes. The most common methods involve the use of alum, talc, anionic pitch-control agents such as polynaphthalene sulfonates or modified lignosulfonates, cationic pitch control agents such as polyquaternary ammonium polymers, and nonionic surfactants. An example of the use of polyquaternary ammonium polymers as pitch control agents can be found in U.S. Pat. No.3,582,461 issued to S. A. Lipowski et al. The patent discloses the use of water soluble dicyandiamide-formaldehyde condensates to prevent pitch deposition on machinery used in pulp and papermaking processes. Examples of attempts to control pitch with other types of compounds or processes are found in U.S. Pat. Nos. 3,812,055; 3,895,164; 3,896,046; 3,992,249; 4,313,790. None of the above methods or compositions is completely effective in preventing pitch from depositing on machinery used in the pulping and papermaking processes. None of the above methods or compositions is completely effective in preventing pitch from depositing in the final paper produced in the pulping and papermaking processes. None of the above methods or compositions is effective at all in removing pitch deposits from machinery used in the pulping and papermaking processes during the operation of these processes. Pitch deposits are normally removed from the machinery used in the pulping and papermaking processes during shut down periods when the machinery is not in actual operation.
Zirconium chemicals have been used by the paper industry since the early 1960's principally as insolubilizers for binders in paper coatings. These coatings are applied to various paper grades such as offset, labels, and food packaging board to impart water resistance during printing or end use. The binders used to adhere the coatings to the sheets normally have poor water resistance and hence insolubilizers are required. Zirconium chemicals have also been used as immobilizers for aqueous coatings to prevent the migration of the binder into the interior of the sheet or to the surface from the interior. For example, when starch is used as a binder in an aqueous coating formulation, it tends to migrate towards the surface of the sheet if the sheet is dried too rapidly before the immobilization point of the starch is reached. The zirconium compounds, most notably ammonium zirconium carbonate (AZC), interact weakly with the hydroxyl groups of the starch forming a type of a network that is water insoluble. The use of zirconium(IV) compounds in the control of stickies is known in the art and has been described in the literature (Tappi Proceedings, 1987 Pulping Conf. p585). Stickies are certain deposits caused by organic materials used in paper and board converting operations and introduced into paper machine furnishes with recycled fibers also called secondary fiber. The word "stickies" is derived from the fact that the deposits cause sticking on wires, felts, and other parts of paper machines. Stickies are a diverse mixture of synthetic polymeric materials ranging from hot-melt and pressure-sensitive adhesives to binders and coatings for inks or wet strength resins. The polymeric materials range from polyethylene, to polybutadiene-styrene, to polyvinylacetate, to polyacrylates, just to name a few. It is believed that the zirconium compounds control the effects of the stickies by reducing the tackiness through the reaction of the hydrolyzed zirconium(IV) ion with oxygenated functional groups on the polymers that make up the stickies. The hydrolyzed zirconium(IV) ion crosslinks hydroxyl- and carboxyl-containing polymers through reaction with these functionalities to produce stickies particles that are less tacky. The reduced tack probably results from the agglomeration of small particles through reaction of surface oxygenated moieties and AZC.